With the increasing availability of efficient, low cost electronic modules, portable communication devices are becoming more and more widespread. A portable communication device includes one or more power amplifiers for amplifying the power of the signal to be transmitted from the portable communication device.
With the decreasing size of portable communication devices, power efficiency is one of the most important design criteria. Reducing power consumption prolongs power source life and extends stand-by and talk time of the portable communication device. In a portable communication device that uses a non-constant amplitude output (i.e., one that modulates and amplifies both a phase component and an amplitude component), a linear power amplifier is typically used. The efficiency of the power amplifier decreases rapidly as the transmission output power decreases from a maximum level. This results in a paradox. To reduce power consumption, the power output of the power amplifier is reduced when conditions permit. Unfortunately, power amplifier efficiency rapidly decreases as the power output is reduced, thus leading to increased power consumption, and reduced power source life.
One type of power amplifier is referred to as a “supply voltage controlled” power amplifier. This power amplifier methodology varies the power output of the power amplifier by controlling the supply voltage to the power amplifier. The output power of a supply voltage controlled power amplifier (PA) is determined by a regulated voltage applied to the collector terminal of a bi-polar junction transistor (or drain terminal, if implemented as a field effect transistor (FET)) of one or more stages of the power amplifier. If implemented using bi-polar technology, this power amplifier is also referred to as a collector voltage amplifier control (COVAC) power amplifier.
To improve the efficiency of a supply voltage controlled power amplifier operating at a low power output level, a switching voltage regulator can be implemented to provide the supply voltage to the power amplifier. Unfortunately, a switching voltage regulator can inject noise and spurious components onto the transmit signal. The control bandwidth of a switching voltage regulator must also be capable of operating over the bandwidth of the transmit signal.
Therefore, it would be desirable to control the voltage applied to a supply control port of a power amplifier to minimize noise, spurious signal generation and switching transients, thereby minimizing spectral regrowth and minimizing power consumption.